Conventionally, a working-gas circulating type gas engine in which fuel gas (for example, hydrogen), oxidizer gas (for example, oxygen), and a working gas (for example, inactive gas) are supplied to a combustion chamber and the fuel gas is burnt while the working gas in the burnt gas discharged from the combustion chamber is circulated to the combustion chamber through a circulation passage has been proposed. It is desirable that the above-mentioned working gas is inactive, as well as has a large ratio of specific heats and raises the thermal efficiency of the engine. Therefore, as the above-mentioned working gas, for example, helium, neon, argon, etc., which are the single atomic gas belonging to rare gasses, can be exemplified. Actually, as a working gas of such an engine, argon is widely used among these.
In the burnt gas of such an engine, the combustion product produced by combustion of fuel gas (for example, H2O (water vapor), CO2 (carbon dioxide), etc.) and working gas (for example, argon, helium, etc.) are contained. Among these, especially water vapor is gas of a three-atom molecule and its ratio of specific heats is smaller as compared with not only inactive gases of a single atom, such as argon and helium, but also air or nitrogen (refer to FIG. 1). Therefore, when exhaust gas containing water vapor with such a low ratio of specific heats is re-supplied to a combustion chamber as it is, the ratio of specific heats as the whole working gas will fall, and the thermal efficiency of the engine will fall as a result. Therefore, the above-mentioned engine generally includes a circulation passage for circulating (re-supplying) the burnt gas discharged from a combustion chamber to the combustion chamber therethrough, and a combustion product removing means (for example, a condensation machine, adsorption material, etc.) interposed in the circulation passage to separate and remove the combustion product (for example, H2O (water vapor), CO2 (carbon dioxide), etc.) contained in the burnt gas.
A working-gas circulating type gas engine is a so-called kind of “a closed cycle engine”, the airtightness of a circulation passage, through which the burnt gas discharged from a combustion chamber is circulated (re-supply) to the combustion chamber, needs to be maintained. If a leak arises in this circulation passage, the circulating gas containing working gas will leak out into the atmosphere, or the air in the atmosphere will flow into the circulation passage conversely. Since nitrogen (and oxygen) which is the main component of air has lower ratio of specific heats as compared with gas of inactivity single atom (for example, argon) used as working gas as shown in FIG. 1, if air flows into a circulation passage, the ratio of specific heats as the whole circulating gas falls, and there is a possibility that the temperature and pressure in the combustion chamber near a top dead center (TDC) may decline and/or the thermal efficiency of the engine may fall, and thereby normal operation may become impossible.
Therefore, in a working-gas circulating type gas engine, it is necessary to quickly and precisely judge the existence of an airtight failure in a circulation passage of burnt gas. In the art, for example, there is an approach to detect leak of gaseous fuel by closing the cutoff valve disposed in a fuel supplying passage which communicates a fuel tank in which the gaseous fuel is stored and a fuel supplying means on the side of an engine in a hybrid car (HV) and detecting the pressure between the cutoff valve and the fuel supplying means by a pressure sensing means (for example, refer to PLT 1).
However, the conventional technology as mentioned above is intended to detect leak in supplying system of gaseous fuel, there is sufficient pressure difference from the open air originally, and therefore it is easy to ensure of the detectability of leak of gaseous fuel. However, in the circulation passage of a working-gas circulating type gas engine as mentioned above, unless special countermeasures are taken, it is difficult to expect sufficient pressure difference from the open air, and it is difficult to detect the leak of circulating gas out of a circulation passage into the atmosphere and/or, conversely, the invasion of the air in the atmosphere into a circulation passage.
There is another approach in the art to conduct an airtight inspection even in the configuration in which it is difficult to expect sufficient pressure difference from the open air unless special countermeasures are taken as described above. For example, there are approaches to close the evaporated fuel supplying system (including a canister) from a fuel tank in which liquid fuel is stored to an air intake system of an engine by a cutoff valve, pressure the enclosed space with a pressurization pump, detect the change of the pressure in the enclosed space with a pressure sensor, and thereby judge the leak in the enclosed space (for example, refer to PTL 2 and PTL 3).
However, in the conventional technology as mentioned above, in order to attain sufficient pressure difference from the open air to ensure the detectability, a pressurizing means such as a pressurization pump is needed. Moreover, when a pressurization pump is disposed in the passage which communicates a canister and the external world, there is a possibility that pressure loss in the passage may result from the exhaust gas at the time of fuel supply and the air intake at the time of purge processing of evaporated fuel going via the pressurizing means or a foreign substance (dust, water) may be mixed in a pressurizing means in association with exhaust at the time of fuel supply or intake at the time of purge processing of evaporated fuel. For example, in PTL (Patent Literature) 2, in order to prevent such inconvenience, a bypass between a passages which communicates a canister and the external world and a passage which communicates the canister and a fuel tank, and a pressurizing means is interposed in the bypass.
That is, in the conventional technology as mentioned above, it is necessary to newly dispose a pressurizing means, a bypass, etc. and, as a result, it lead to causing enlargement and complication of the system containing such a working-gas circulating type gas engine, and increase in cost. As mentioned above, in the art, there has been a continuous demand to a brief technology for detecting an airtight failure in a working-gas circulating type gas engine with sufficient accuracy and at low cost.